Triple walled underground storage tank

ABSTRACT

A triple-walled underground storage tank is provided with primary, secondary and tertiary walls. Between each two adjacent walls, an annular space is provided. The annular space is continuous throughout, and permits the flow of liquid therein. Monitors may provided separately for each annulus, to detect the formation of leaks, or a lack of containment, in either wall defining the annular space. In one embodiment, the tertiary wall is a smooth cylinder formed about the circumferential ribs of the secondary wall. In the alternative embodiment, the rib-bearing wall is the tertiary wall, and the walls and annular materials of the tank are formed within a female mold. In either case, a tank prepared from integral ribs, with superior robustness and strength, is provided with triple containment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to underground storage tanks for the storage ofliquids in general and in particular chemicals, and most particularly,fuels such as gasoline, diesel fuel, heating fuel and the like, as wellas petroleum products such as machine and lubricating oils. These tanksare adapted to contain liquids underground, and dispense them, through apump, to a distant location, such as the nozzle of a gasoline pump at agas station. Specifically, the invention contemplates a triple walledtank, with an inner or primary wall, a secondary wall and an annulusbetween the primary and secondary wall, and an outer or tertiary wall,with an annulus between the second and tertiary walls.

2. Background of the Prior Art

Commercially, underground storage tanks have been in use in the UnitedStates for over on hundred years. With the development of a nationwideroad system, and reliance on the automobile and related transportation,gasoline service stations were widely develop. Although not the solesite for use of underground storage tanks, typically, gasoline at aservice station is held in underground storage tanks and dispensedthrough above-ground facilities. This invention focuses on the structureof these underground storage tanks, which can also be used to storechemicals and other liquids, but find a principal application in thestorage of liquid fuels.

Initially, storage tanks were constructed of steel. The use of steelpresents a problem of corrosion, however, with failure of containmentlikely over a long term of service. This failure became even more likelyif the hole where the tank was situated was a “wet” hole, that is onefilled with water, or typically, brine, a highly corrosive fluid. Thisproblem spurred the development of corrosion-resistant tanks constructedof fiberglass and resin, generally referred to as fiber reinforcedplastic, or FRP. Xerxes Corporation, a leader in fiberglass reinforcedplastics, and its predecessor, introduced FRP underground storage tanks(storage tanks, herein) in the 1970's, along with Owens CorningFiberglass.

While FRP tanks provide resistance to corrosion, failure of any tank,due to mechanical weakness, impact, etc. presented the possibility ofenvironmental contamination of the area surrounding the storage tank, aswell as an expensive cleanup. A response to this concern was theintroduction of the “double walled” FRP tank, in which two concentricwalls, separated by an annulus in which some form of leak detectionmonitor was provided, were employed to provide back-up protection.Leakage in either the inner or outer wall was detected through an alarmmeans, or monitor, and provided the opportunity to locate and repairdamage before penetration of both walls could occur. The first suchdouble wall FRP tank was commercially introduced by Xerxes Corporationin 1984.

Double wall storage tanks, particularly of FRP construction, have sincereceived substantial attention in the patent literature. Among patentscommonly assigned herewith are U.S. Pat. No. 5,544,974, directed to anoptimized service station installation, and U.S. Pat. No. 5,595,456,directed to the provision of a water-tight riser for a double walled FRPstorage tank.

U.S. Pat. Nos. 5,220,823; 4,988,447 and 4,974,739; address double walledunderground storage tanks where the annular space between the inner andouter wall provides flow through of liquids therein, but provides forsome strength sharing between the two walls. U.S. Pat. Nos. 5,020,358;5,017,044; 4,825,046; 4,875,361 and 4,739,659, all commonly assignedherewith, address different solutions to the need to provide for dualcontainment of the interior fluid, monitoring of the integrity of thetanks, and ease of construction.

Typically, a double walled FRP storage tank is built using either “male”or “female” construction. In the male method, the tanks is formed abouta rotating mandrel, with a layer of FRP material formed on the outersurface of a cylindrical mandrel. Reinforcement ribs, to provide hoopstrength, and resistance to buckling, are formed on the first or primaryFRP layer, and adhered thereto through a secondary bond. In a doublewalled tank, a layer of annular material is placed over some or all ofthis rib bonded primary wall, and then a secondary wall of FRP materialis formed on top of the annular material. Although originally, thesecond wall touched the tops of the ribs and spanned the distancetherebetween, spaced from the region of the primary tank between theribs, or “flats”, as illustrated in U.S. Pat. Nos. 4,781,777 and4,679,093, this created an annulus of substantial volume. As a “wet”annulus, that is one where the annular space is filled with a leakdetecting fluid such as brine, became increasingly popular, the weightof the annular fluid in these tanks became significant, and a contourfollowing tank was developed, where the annular material, and thesecondary containment tank, follow the general contour of the primarytank ribs and flats.

In the female method, the tank is formed inside of a mold, rather thanon a mandrel. The mold has ribs provided in it, giving rise to amonolithic or primary bond between the flats and the ribs—they areco-formed and co-cured. This gives rise to a more robust tank, withsubstantial structural integrity. In nearly thirty years of commercialuse, not one tank of this type, manufactured by Xerxes Corporation, hasfailed due to lack of mechanical strength. In this system, the annularmaterial is applied to the inside of the outer wall, and then thesubstantially cylindrical inner wall is formed on the inside of theannular layer. U.S. patent application Ser. No. 08/705,765, allowed,discloses this method of tank construction, where the annular materialis reduced to a Mylar film, or PVA and wax in the dome soaped ends. Theentire disclosure of this application is incorporatedherein-by-reference.

Recently, increasing population pressure, and recognition of thesensitivity of sources of water to environmental pollution, have givenrise to requests from government and private interests to provide evengreater security than those offered by commercially available doublewalled underground storage tanks, including FRP tanks, currentlyavailable from Xerxes Corporation and Containment Technologies.Enhanced, or triple wall protection, has been requested by various localgovernments, including those concerned about the safety of the SanAntonio, Tex. aquifer, and San Francisco, Calif. Currently, no triplewall tank is available, nor is a double wall tank readily available tobe combined with existing technology to provide such triple walltechnology.

SUMMARY OF THE INVENTION

The above-stated goals, including the provision of a triple walled tankfor additional security of containment of fluids within the primarytank, is provided by combining technologies proven over time in separatetanks, so as to provide an internal or primary tank, with a secondarytank surrounding the primary tank, and an annular material spacing thetwo apart. A tertiary tank is placed surrounding the secondary tank, andagain, an annular space is created between the secondary and tertiarytanks.

The structure can be provided in either of two embodiments. In the firstembodiment, the primary and secondary tank, with the annulus therebetween, are built in a fashion identical to that disclosed in U.S.patent application Ser. No. 08/705,765. Specifically, the outer orsecondary tank is formed on the interior of a female mold, with a mylarfilm or other separating material applied along the flats of theinterior of the secondary tank. A layer of unidirectional fabric, or“uni” is placed over the open ends of the ribs formed in the secondarytank, and coated with FRP materials, to seal the rib. In at least threepoints around the circumference of the tank, “gutters” of dimensionallystable material which permits the flow of liquid longitudinally areinstalled, connecting each annular pace and the space within each rib,by punching holes in the overlaying material. Thereafter, the interior,or primary tank of FRP materials is “sprayed up”, or formed on theinterior of the annular material. Each tank is formed from twohalf-shells, each with a closed, dome-shaped end. The primary andsecondary tanks are separated, in the dome, by application of apolyvinyl alcohol (PVA)/wax combination, insuring the flow of liquidbetween these walls in the dome. The two half shells are then married,and secured by an overlay of FRP material on either side of the joint.

In a first embodiment, a tertiary wall is provided with an annulusbetween the secondary wall and itself, by forming a smooth cylinderwhich rests on, and in preferred embodiments, is secured, to the tops ofthe ribs of the secondary containment wall. This tertiary wall ispreferably formed by wrapping Bayex® or similar FRP supporting fabricabout the exterior of the tank, from rib top to rib top, and then“spraying over” or applying resin or fiber reinforced plastic thereover, and allowing the same to cure. The tertiary wall defines annularchambers between each adjacent pair of ribs. To provide for fluidcommunication between the chambers, an insert is provided at one or twolocations along the rib, on the top of the rib, providing for flowbetween adjacent annular chambers.

In an alternate embodiment, the double walled half shells, with anannulus there between are not removed from the mold. In this embodiment,the outer most wall formed against the mold interior surface becomes thetertiary wall, and the wall previously formed becomes the secondarywall. A second annular material is applied to the inner surface of thesecondary wall, and secured thereto, typically by providing resin toadhere the annular material to the interior wall by providing a tackysurface. Against the interior of this second annulus, a primary wall isbuilt, creating a half-cylinder in each mold half. Two mold haves arethen married to form a single tank.

In either embodiment, the reinforced ribs, necessary for bucklingresistance and hoop strength, are integrally formed with the wall withwhich they are associated (either the secondary or tertiary wall, in thefirst or second embodiments, respectively). This provides for a strong,robust and durable tank. In preferred embodiments, there is a bondbetween each of the walls, either directly, or through astrength-sharing, load transmitting annular material. Each annulus isprovided with a leak detection system, so that leakage through any ofthe three walls can be detected quickly, and repaired without loss ofcontainment.

BRIEF DESCRIPTION OF THE DRAWINGS

The underground storage tank of this application is more completelyunderstood with reference to the attached drawings.

FIG. 1 is a cross section illustrating the typical wall and annulusarrangement of a first embodiment of the tank, taken in longitudinalcross-section.

FIG. 2 is a longitudinal cross-section of the wall arrangement of afirst embodiment of the tank, illustrating the seam between twohalf-shells.

FIG. 3 is a illustration through a longitudinal cross-section of a firstembodiment of the tank employing gutters, or other longitudinal flowmeans, for providing communication between annular spaces in the flats,and the interior of the ribs.

FIG. 4 is an illustration of the juncture between the dome and cylinderwalls of a first embodiment of the tank of this invention.

FIG. 5 is a plan illustration of a first embodiment of this tank,illustrating the placement of gutters longitudinally along the tank.

FIG. 6 is a cut away view of the exterior of a first embodiment of thetank.

FIG. 7 is an illustration of a manway provided in a first embodiment ofa tank through a longitudinal cross-section.

FIGS. 8 and 9 illustrate liquid monitoring reservoirs to detect leakagein either a flat, FIG. 8, or a rib, FIG. 9 in a first embodiment of thetank.

FIG. 10 is an illustration of a fitting providing in the firstembodiment of the tank.

FIGS. 11A-G illustrate sequential formation of a tank according to asecond embodiment of the invention.

FIG. 12 illustrates a cross-section of a second embodiment of thisinvention, with a leak detection monitor for the primary annulus.

FIG. 13 illustrates a typical cross-section through a second embodimentof the tank, and FIG. 14 illustrates a monitoring device for thesecondary annulus of the tank, co-located with the monitoring device forthe interior annulus.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a triple walled underground storage tankcomprised substantially of fiber reinforced plastic materials.Typically, these materials comprise a curable resin, reinforced byfibers of fiberglass, or similar reinforcement material. Such materialsare conveniently “sprayed” up on a receiving surface, and cure in place,by use of a “chopper” gun. One such device is disclosed in U.S. Pat. No.5,654,231, which is incorporated herein-by-reference. Although operatedby a controller, the process of fabricating underground storage tanksthrough the use of female molding technologies can be semi-automated byuse of an apparatus and process of the type disclosed in U.S. Pat. No.4,363,687, also incorporated herein-by-reference. In general, femalemolding technology calls for the placement of a female mold with adesired configuration of ribs and flats on a roller bed, which rotatesthe mold, which corresponds to a half-shell, at a suitable speed. Theoperator, from within the mold on a platform extended therein, spraysthe mixture of fiberglass and resin, through the chopper gun, on to theinterior of the mold surface. The mold surface is prepared, prior tospraying, with a mold release agent, to provide for ease of separationof the cured product from the mold. In a process, such as thatcontemplated herein, particularly thick layers, such as the wallsthemselves, may be completed through two or more passes, sinceapplication of the entire thickness in one pass will lead to theformation of bubbles and potentially pin hole leaks as the FRP materialcures.

Any of a wide variety of resins may be employed. These typically includepolyester resins, such as vinyl esters and isophthalic polyesters,polyethylene, polypropylene, polyvinyl chloride, polyurethane, epoxy,and other resins. Typically, the material sprayed on also includes acatalyst to induce and accelerate curing. In some cases, the catalystmay be colored, or a colored tracer element introduced with thecatalyst, to ensure thorough distribution of the catalyst in themixture.

Similarly, various grades of fiberglass, as well as other reinforcingfibrous materials, may be employed. Other materials necessary or ofvalue in the fabrication of the underground storage tanks of thisinvention are described below.

Referring initially to FIG. 1, a first embodiment of the invention isillustrated in longitudinal cross-section through a portion of the tankbetween the mid-point, where two half shells are joined, and theend-point, where the tank terminates in a dome-shaped continuous end.Tank 100 is comprised of interior or primary wall 102, which is itselfcontained by secondary wall 106. Between the two is an annular material104. This material may be of any of a wide variety of compositions,shapes and textures, as are disclosed in the art discussed herein. Inone preferred embodiment, the annular material is a thin mylar or otherplastic sheet, as disclosed in allowed U.S. patent application Ser. No.08/705,765. In this embodiment, the annular material between primarywall 102 and secondary wall 106 serves to provide a space between thewalls, wherein liquid flows.

Secondary wall 106 is contained, in turn, by tertiary wall 108. Thespace between tertiary wall 108 and secondary wall 106 is occupied byannulus 114. The annulus may be filled, partially filled or entirelyopen. The only requirements for this annular space, if it is filled, isthat the material therein permit the passage of water or other fluid,including the fluid to be contained within primary tank 102, inreasonable time. Fluid communication between the annular chambers ofannular space 114, defined by adjacent ribs 110, is provided by insert116. This insert is a small piece of rigid material, either plastic ormetal, which is secured to the top of each rib 110 at two or threepoints along the circumference of the rib, which extendscircumferentially all about the tank. This insert 116 is provided withslots which provide for fluid communication between annular chambers114.

Each of walls 102, 106 and 108 are prepared substantially from fiberreinforced plastic materials, although the method of manufacture of wall108 is distinct from that of walls 102 and 106.

The structure comprising walls 102, 106 and annular material 104 isfully disclosed in pending U.S. patent application Ser. No. 08/705,765.The resulting tank, without the provision of third wall 108, iscommercially sold by Xerxes Corporation and generically referred to asthe “DWT-II” tank. In brief, secondary wall 106 is formed against theinterior of a female mold which has a corrugated surface defining ribs110, flats 112 and the spacing there between. This wall may be sprayedup from a mixture of glass fibers and resin, in one or more passes.Thereafter, annular material 104, which may be a mylar film, is appliedto the “tacky” resin surface of the interior of secondary wall 106. Themylar or other annular material 104 serves to separate secondary wall106 from primary wall 102. In an alternate embodiment, a meltable wax orsoluble coating such as a polyvinyl alcohol of low saponification isapplied to the interior face of the flats of secondary wall 106.Thereafter, primary wall 102 is sprayed up on the surface of thiscoating. Subsequent to curing of the primary wall, the coating is meltedor solubilized, leaving annular space 104, which in this embodiment, isempty.

In the embodiment illustrated in FIG. 1, ribs 110 define an interiorspace which constitutes part of the annulus. To maintain the ribs openwhile applying primary wall 102, a layer of unidirectional fabric,typically comprised of fiberglass running the longitudinal directionheld in place by secondary fibers, referred to as “uni”, is adhered tothe “shoulders” of the rib formed in secondary wall 106. FRP material isthen sprayed up on the uni fabric and primary wall 102 sprayed up thereover. The uni fabric serves as a manufacturing aid only, and does notconstitute a structural aspect of the invention. Importantly, in thisinvention, annular space 104 extends to the rib shoulder, to ensure dualcontainment between primary tank 102 and secondary tank 106. Thispermits secondary tank 106 and primary tank 102 to be “bonded” at eachfoot of ribs 110. In fact, this “bond” forms as a co-cure, and resultsin a robust tank of significant strength and durability.

In this embodiment, tertiary wall 108 is formed by removing the tankalready formed from the mold, and applying a supporting fabric or baseabout the exterior of secondary tank 106. In this case, the tertiarytank is not “contour following” but rather presents a smoothcircumference extending the length of the tank. This is most clearlyillustrated in cut away in the cut away view illustrated in FIG. 6 wheretertiary wall 108 is shown enclosing a tank with twenty ribs 110. Whilethe tertiary wall may be formed separately by female or male moldingtechnology, to avoid the need to observe extremely close tolerances,tertiary wall 108 may be formed by wrapping a supporting fabric, such asfiberglass mesh, available under the mark Bayex®, about the ribs 110.Each piece of Bayex® may be conveniently trimmed to extend frommid-point of a first rib 110 to mid-point of the adjacent rib 110. Oncethe Bayex® has been secured in place, it is over sprayed with resin orFRP materials, and allowed to cure. On curing, the tertiary wall 108exhibits some shrinkage, providing for a tight fit. Desirably, however,a secondary bond is provided between tertiary wall 108 and rib surfaces110 at points other than those occupied by flow through device 116. Thisis most effectively achieved by sanding the surface of rib 110 prior toapplication of the Bayex® FRP materials comprising tertiary wall 108.

As noted, an entire tank is comprised of two half shells, most clearlyillustrated in FIG. 2. On completion of the double walled tank comprisedof primary wall 102, annular material 104 and secondary wall 106, in twohalf shells, the tank is removed from the half shells, and two halvesbrought together on a bed which rotates the shell. An inner seam tablay-up 118 is applied over the junction between the two half shells. Thehalf shells are married carefully to maintain a continuous annular space104. The lay-up consists of a fiberglass mat, or glass roving, to whichcurable resin is applied. As illustrated, the seam lay-up is provided onboth the interior and exterior of the tank as formed. Access to theinterior may be provided through a manway, discussed, infra. Overlay-ups 118 a seam 120 is provided, again comprised of a glass mat orrovings, provided with resin. Thus, all the structural materials of thetank are comprised of corrosion-resistant FRP-type materials.

In order to provide for communication through all the rib and flatspaces defining the annulus between primary tank 102 and secondary tank106, gutters are provided, as most clearly illustrated in FIG. 3. Theannular flow material may be any dimensionally stable material thatprovides for, or allows, liquid to flow longitudinally therein. Thismaterial may be mylar, or more rigid materials available to those ofskill in the art. In one embodiment, thermal plastic 3-dimensionalwebbing, produced under the mark Conwed®. Similar materials are alsoavailable from Qualis Corporation, a company of Kentucky. As illustratedin FIG. 2, this annular flow material 122 extends along the flat and therib, linking he spaces contained within ribs 110 with the annularchambers defined by annular material 104. A hole indicated at 124 ispunched through the “uni” material described above, so as to ensurecomplete liquid communication between the space within ribs 110 and theannular space 104. Typically, three gutters are provided, at spacedlocations running longitudinally along the tank. The spacing of thesegutters is illustrated in FIG. 5. Typically, two gutters will beinstalled within 20° or so of the “top” of the tank, that portion of thetank which will be provided with fitting extending upwardly when thetank is installed. These gutters 122 a are generally narrower than thethird gutter which is aligned along the bottom of the tank, indicated at122 b.

As noted above, the underground storage tank of this inventionterminates, in a dome-shaped manner, at both ends. The junction of thelast circumferentially extending rib with the structure of the dome ismost clearly illustrated in FIG. 4. Terminal rib 110 again overlays a“gutter” or annular flow material strip 122. This communicates with theannular space between primary tank 102 and secondary tank 106,particularly through annular material 104. In the dome, the mylar filmor other annular material is replaced with a coating of PVA combinedwith wax, which coating is removed upon filling the annular space withbrine, water or any other liquid. Gutter 122 extends to this point, sothat a continuous annulus with fluid communication throughout, extendsfrom one end of the tank to the other. This is important, should a leakof any type develop either in primary tank 102 or secondary tank 106. Byfilling the annular space there between with brine or some othermonitoring fluid, potential leakage can be quickly detected, located andrepaired. Even if there should be leakage through both walls 102 and106, any leakage is contained by tertiary wall 108. Liquid monitoringsystems may employ special reservoirs, discussed infra. Instead of aliquid monitoring system, a dry monitoring system may be employed. Sucha system may be comprised of a sensor sensitive to the presence ofliquid in an annulus which is otherwise intended to be dry, or apressure system, wherein a low degree of vacuum is drawn on the annulus,which vacuum will be broken upon the formation of leaks or holes in anyone of the tanks. The annular space between primary wall 102 andsecondary wall 106 is monitored by one device, and a separate devicemonitors the annulus between tertiary wall 108 and secondary wall 106.

In many tanks of diameter 6 foot or larger, access to the interior isdesired, and is provided by a manway. The construction of such a manwayis illustrated in FIG. 7. In this illustration, a hole is cut betweenadjacent ribs 110. The hole is desirably large enough to permit easyaccess and egress for an adult male. The manway opening is generallyindicated at 130. The opening is sealed by manway wall 132, againcomprised of FRP materials and constructed on a mold, to form the manwaycollar. In the alternative, it may be formed in situ, against atemporary form. Manway collar wall 132 is secured to tertiary wall 108,secondary wall 106 and primary wall 102 by the application of lay-ups offiberglass mat or woven roving, or similar fiberglass fabric,impregnated with resin, providing a corrosion-resistant FRP seal at eachjoint 128. To ensure there is no leakage or separation at the corners oflay-ups 128, which occur continuously around manway collar 132, putty134 is installed at the corners. In this way, access to the interior ofthe tank, without interrupting the triple wall nature of protection, orthe annular flow, can be provide.

If either annular space 104 or annular space 114, is to be filled, thatis a wet annulus separate structures have to be provided to providereservoirs for the brine or other detecting monitor liquid to beprovided. A reservoir for annulus 114 is illustrated in FIG. 8. Annulus114 is of substantial volume in a large tank, and a reservoir isprovided so as to adjust for fluctuations in the volume due to thepassing of trucks or other loaded vehicles above the tank, the influenceof temperature and pressure, etc. Typically, a float valve or otheralarm means can be installed within the reservoir, such that either adrop or significant increase in the level in the reservoir triggers analarm, which can be electronically communicated through the hole infitting 140, to a remote site which monitors the integrity of the tank100. Reservoir wall 148 is formed separately on a mold, and applied tothe outer surface of tertiary tank 108. As illustrated in FIG. 8, thisreservoir spans two ribs 110, but the size will be adjusted to the sizeof the tank, and the height of the rib, which determines the volume ofannulus 114. As with the manway, reservoir wall 148 is bonded to outerwall 110 by lay-ups of glass mat and woven roving, which are impregnatedwith resin. Again, at corners, to ensure tightness of fit, putty 134 isinstalled. Holes 144 are provided through tertiary wall 108, tocommunicate with annulus 114. At the top of the reservoir, a fitting 140is provided on mounting plate 136 which is secures resin impregnated matboth below and above the mounting plate at 138 and 142. This processgenerally referred to as “glassing in”. Annulus 114 and a portion of thereservoir illustrated in FIG. 8 are conveniently filled through fitting140, which can then be provided with the necessary fixtures to carryelectrical contacts for any monitor provided, and any other apparatusdesired.

A similar reservoir, for annulus 104, is illustrated in FIG. 9, wherelike characters indicate like materials. Thus, a fitting 140, mountingplate 136 and mat and lay-ups 138 and 142 are provided identically. Aswith the reservoir for annulus 114, the reservoir for annuls 104 isadhered to outer wall 108 and inner wall 102 by wet lay-ups of resinimpregnated glass fabric. At the comers, putty 134 is provided. Thereservoir illustrated in FIG. 9 sits atop a rib cap, so thatcommunication is directly with the interior of the rib, and therebythrough the gutters with annular spaces throughout the tank betweenprimary wall 102 and secondary wall 106. The connection between thereservoir and primary wall 102 is shown in phantom.

In addition to manways and reservoirs, tanks come provided with variousfittings to permit the insertion of and communication with fill pipes,pump lines, pumps, vents, monitoring means and the like. A typicalfitting installation for this embodiment of the invention is shown inFIG. 10. The installation of fitting 152 is essentially identical,except for size, to the application of the fittings to the reservoirsillustrated in FIGS. 8 and 9, including mounting plate 136 and lay-ups138 and 142. The fitting is walled off by fiber reinforced plastic wall150, which connects secondary wall 106 with tertiary wall 108.Fiberglass mat, impregnated with resin, is wrapped around primary wall102 up the side of fitting wall 150, and then a similar mat lay-up 148 bis wrapped over tertiary wall 108 and lay-up 148 a, thereby effectivelysealing off annulus 114 and annulus 104.

In an alternate embodiment of this invention, the tank is preparedentirely within the female mold, as two halves closed by a dome-shapedend and remaining, at the opposite end, open. In this case, the outermost wall becomes the first wall formed within the mold, and theexterior of the tank has a “corrugated” appearance caused bylongitudinally spaced, circumferentially extending ribs. Because thetank structure is built inward, rather than outward, the interior of thetank is of slightly reduced diameter. This can be compensated for, toprovide equal volume, by lengthening of the tank.

FIGS. 11A-G illustrate the formation of sequential layers of the tank ofthis embodiment of the invention. Because of the difference inappearance of this embodiment of Applicant's triple walled tank,different reference characters are applied to seemingly similarly,structures. In both cases, however, a triple walled tank, with annularspaces between each two adjacent walls which permit the flow of liquidtherein, is provided.

The tertiary and secondary walls of the tank, and annular space therebetween, together with the ribs therefore, in this embodiment of thetank may be built in a fashion identical to the first embodimentdescribed above, and that disclosed in U.S. patent application Ser. No.08/705,765. Thus, tertiary wall 202 is first formed, in a shapecorresponding to the interior of the female mold, with ribs 204 andflats 206 integrally formed, preferably in a plurality of passes.Thereafter, annular material, which may be a mylar sheet, is applied tothe flats, and subsequently, a uni layer 210 used to close off thebottom of rib 204. On the annular material and uni layer 210, secondarywall 212 is formed by spraying FRP materials against these layers.Optionally, the annular material may be a force transmitting, strengthsharing three-dimensional fabric which permits the flow of liquidtherein, as described, infra.

At this point, rather than removing the tank from the mold, the tank isleft inside the mold. Importantly, at this point, it is necessary toperform quality control to ensure no pin holes or delaminations haveoccurred. In alternative A described above, inspection can be easily andquickly done by “soaping” the inside and outside of the tank, applying asmall degree of pressure, and detecting leaks or holes by bubbling.Soaping the inside of the tank within the mold is a difficult procedure.In one alternative, ultrasound, or other sound testing, can be used todetect inconsistencies in layer thickness, pinhole formation, ordelamination. In an alternative embodiment, the open end of the halfshell formed within the mold can be closed by application of a layer ofFRP material, and a light vacuum drawn against annulus 208 by cutting asmall hole in secondary wall 112 and applying vacuum there through. Ifthe vacuum holds, no pinhole formation or delamination is observed. Theclosing layer of FRP is then removed.

After quality control has been completed, a second annular material 214is applied. This may be a mylar film as described above. In thealternative, it may be a flow through material which provides for somestrength sharing between the two walls, such as that disclose in U.S.Pat. Nos. 5,020,358 and 5,017,044, incorporated herein by reference. Inparticular, a needled felt, such as that made available by OziteCorporation under the mark Compozitex™ is suitable.

In an alternate embodiment, a three-dimensional fiberglass reinforcedplastic, which “swells” or expands upon resin impregnation may be used,which material forms and cures, adhering to secondary wall 212. In thisembodiment, annular material 214 is similar to that made a available byParabeam, as set forth in U.S. Pat. No. 5,534,318. This technology wasoriginally developed in the United States by Xerxes Corporation. Theentirety of the disclosure of U.S. Pat. No. 5,534,318 is incorporatedherein by reference. Similar “three-dimensional reinforcing fabric,which is comprised of double pile cloth having spaced apart top andbottom cloths and binding threads there between”, is available fromVorwerk of Europe. One suitable material is offered under the MarkTechnoTex. Another material that may be used is Flocore, as described inU.S. Pat. No. 5,522,340. Each of these materials is adhered to theinterior of secondary wall 212 by resin, providing annular material 214.Primary wall 216, of FRP materials, is then sprayed up on the interiorof annular material 214. The resulting structure provides atriple-walled “sandwich” with annular spaces provided between each twoadjacent walls.

The completed tank, illustrated at the juncture with the dome, is setforth in FIG. 11G. Thus, tertiary wall 202, provided with integral ribs204 provides outside containment. Rib 204 is sealed off with uni layer210 and annular material 208 provides fluid communication throughout theannulus between tertiary wall 202 and secondary wall 212. Annularmaterial 214 lies between secondary wall 212 and primary wall 216, eachof the walls being formed of FRP material. In place of the mylaremployed, annular material 208 may also be a three-dimensional fabric,or other “strength-sharing” material which permits fluid flow therewithin. This is particularly in light of the fact that the hoop strengthprovided by ribs 204 can be more effectively communicated by such anarrangement, given that there is no co-cure or integral bond betweentertiary wall 202 and secondary wall 212. As with the first embodimentdiscussed above, at point 220, the annular material 214 overlaps withthe PVA/wax annular material at 222 a and 222 b, for both the inner andouter annulus at the dome. To provide improved communication,unidirectional glass may be applied over the terminus of annularmaterial 214, so as to ensure at the PVA/wax combination does notpenetrate the three-dimensional fabric.

Because both the annular space between the primary and secondary wall,and that between the secondary and tertiary wall, are quite small, bothcan be conveniently filled with brine or other liquid monitoring fluid.A monitor is provided in a fashion similar to that illustrated for thefirst embodiment described above. A monitoring fixture for monitoringthe fluid level in annulus 214, between primary wall 216 and secondarywall 212 is illustrated in FIG. 12. Reservoir 224, formed of FRPmaterials, is made on a separate form and attached to tertiary wall 202,preferably at a rib. As before, a lay-up of fiberglass mat impregnatedwith resin is applied at 232 to bond the reservoir with the remainingrib. In this embodiment, an FRP disk 226 is provided, pre-fabricated,with a hole in the center to receive fiberglass coupling 228. Thecoupling 228 is also secured by lay-ups of resin impregnated mat 230,and provided with putty seam 234. The coupling 228 clearly communicateswith the annulus 214. A dual-monitor, which also provides communicationwith annular space occupied by annular material 208 is shown in FIG. 14.In this case, a second monitor in coupling 236 is provided, which sitsatop, and fits on, monitor 228. As shown, this communicates with thespace within rib 204 and thus with the continuous annular space dividedby annular material 208. The provision of a second monitor of this typemay require a doghouse of greater height, which can be convenientlyprovided by extended top 240. As before, this is glassed in with wetlay-up 238.

A representative cross-section of this tank is illustrated in FIG. 13.The principle elements of the tank include outer wall 202 andcircumferential rib 204. As illustrated in FIG. 13, both annulus 208 and214 are made of materials which adhere to their adjacent walls,permitting the transmission of force, and strength sharing, therebetween. Thus, tertiary wall 202 overlays annular material 208, which issandwiched by secondary wall 212. Annular material 214 is applied to theinterior wall 212, with primary wall 216 being applied to the inwardface of annular material 214.

The tanks of this invention may be installed singly, but are preferablypart of a multiple installation. While tanks of superior strength anddurability are provided, care and handling in installation, aspectswhich are beyond the scope of this invention, must be maintained, aswith all underground storage tanks. The first embodiment of this tankprovide for ease of manufacture and quality control. The secondembodiment of the invention provides for entirely “in-mold”construction, and an annulus between the tertiary and secondary walls ofreduced volume, making filling a the manufacturing site, rather than theinstallation site, possible. Both alternatives offer advantages whichmay be selectively employed, depending on the application of the tank tobe made.

The invention has been described generically, and with reference toparticular materials, shapes and structures. Alternatives will occur tothose of skill in the art, particularly in the selection of annularmaterials, specific resins and fiber reinforcement, and the like,without the exercise of inventive faculty. Such alternatives remainwithin the scope of the invention, unless specifically excluded by theclaims set forth below.

What is claimed is:
 1. A triple-walled underground storage tank (UST)comprising: (a) a generally cylindrical primary wall adapted for thecontainment of liquid materials, terminating in two dome-shaped ends;(b) a secondary wall encircling substantially the entirety of saidprimary wall and providing secondary containment for said liquidmaterial, wherein a first annulus is provided between substantially theentirety of said primary and secondary wall through which liquid mayflow; and (c) a tertiary wall encircling substantially the entirety ofsaid secondary wall and providing tertiary containment for said liquidmaterials, wherein a second annulus is provided between substantiallythe entirety of said tertiary and secondary walls through which liquidmay flow; (d) wherein said first and second annulus are not in fluidcommunication with each other and a space interior to the primary wallwherein said liquid materials are contained; and (e) wherein therelationship between the primary wall and the second wall is such thatfluid flow is permitted through substantially the entire first annulus,and the relationship between the secondary wall and the tertiary wall issuch that fluid flow is permitted through substantially the entiresecond annulus and wherein at least one of said first and second annulusis substantially filled with monitoring liquid and said primary wall isbonded to said secondary wall and said tertiary wall is bonded to saidsecondary wall.
 2. The triple-walled underground storage tanks of claim1, wherein said primary, secondary and tertiary walls are comprisedprincipally of fiber reinforced plastic (FRP).
 3. The triple-walledunderground storage tank of claim 1, wherein said first annulus and saidsecond annulus are provided with separate monitors for determining theformation of leaks in said walls.
 4. The triple-walled undergroundstorage tank of claim 3, wherein said monitors comprise reservoirsformed of FRP and adhered to said tertiary and secondary walls.
 5. Thetriple-walled underground storage tank of claim 1, wherein said tank isbuilt wholly within a female mold, and said tertiary wall is comprisedor ribs formed integrally with said tertiary wall.
 6. The triple-walledunderground storage tank of claim 4, wherein said first and secondannulus are comprised of materials adhered to said walls on either sideof said annulus, and through which fluid may flow.
 7. The triple-walledunderground storage tank of claim 6, wherein said annular material of atleast one of said first and second annulus is comprised of athree-dimensional fiberglass fabric impregnated with resin.
 8. Thetriple-walled underground storage tank of claim 7, wherein both saidfirst and second annular materials are comprised of three-dimensionalfabric comprised of fiberglass impregnated with resin.